Recently, more transmission capacity and higher transmission speed has been required for data transmission systems. For satisfying such requirements, using the optical WDM (Wavelength-Division Multiplexing) technique is suitable for such systems. An optical cross-connect apparatus is one key device for developing such a system.
However, a large-scale optical switch is needed for such optical cross-connect apparatus. For example, suppose that a number of input/output optical fibers as N, and a number of optical multiplexing on each input/output fiber as M, the required switch size becomes MN.times.MN. Here, MN is the value which carries out the multiplication of the number N of input/output optical fibers to number M of optical multiplexing on each input/output optical fiber.
Moreover, the switch in such an optical cross-connect apparatus should be switched rapidly in the case of system failure or line failure, so as to achieve a quick restoration.
For example, a LiNbO3 optical waveguide switch using the electro-optic effect can be used as such a switch in which high-speed operation is possible. Also, a glass optical waveguide switch using the thermal-optic effect can be used.
Typical conventional large-scale optical switches are constructed by connecting several stages of small-scale unit switches. For example, Nakajima et. al, "Prototype WP-Based Optical Path Cross-Connect System Using PI-LOSS Switches", Technical Report of IEICE, OCS98-29 (1998.7.23), discloses an optical cross-connect system using an 8.times.8 optical switch. The 8.times.8 optical switch in the cross-connect system has a plurality of unit switches.
The system described in the document demultiplexes the optical wavelength multiplexed signal on each transmission line. The system then inputs the demultiplexed optical signal into the unit switches of the 8.times.8 optical switch. Here, optical signals of the same wavelength from each transmission line are inputted to the same unit switch. The unit switch switches the inputted optical signal and then applies it to the wavelength multiplexer, which corresponds to an output transmission line. At the wavelength multiplexer, the inputted optical signals are multiplexed and then applied to the output optical fiber.
However, the optical switch concentrates the optical signals of the same wavelength into the same unit switch. As a result, the optical switch cannot convert the wavelength of an optical signal into a different wavelength. For example, an optical signal of the wavelength .lambda.1 of the transmission line #1 can be converted to the optical signal of the wavelength .lambda.1 of the transmission line #2. In contrast, an optical signal of the wavelength .lambda.1 of the transmission line #1 cannot be converted to the optical signal of the wavelength .lambda.2 of the transmission line #2. Moreover, the optical switch cannot output a plurality of optical signals of the same wavelength from different transmission lines into the same transmission line.